A CRT is evacuated to a very low pressure and accordingly is subject to the possibility of implosion due to the stresses produced by atmospheric pressure acting on all surfaces of the CRT. This problem has been addressed in the art by providing the CRT with an implosion protection band. Such a band is used to apply compressive force to the sidewall of a faceplate panel of the CRT to redistribute some of the forces. The redistribution of forces decreases the probability of a spontaneous implosion of the tube by minimizing tension in the corners of the panel. An implosion protection band also is beneficial because it improves the impact resistance of the tube. Glass in compression is stronger than glass which is in tension, and the band causes compression in panel areas which otherwise would be in tension. The implosion protection band also provides a convenient structure on which to attach a degaussing coil that is used to re-magnetize internal ferromagnetic components of the CRT and mounting lugs to secure the CRT within a cabinet or receiver.
It is known that the magnetic field of the earth affects the paths of the three electron beams emitted by a CRT electron gun and may cause what is called "misregister" in a color CRT. A color selection electrode, such as a shadow mask, having a multiplicity of openings therethrough, is located in proximity to a tricolor screen of the tube to assure that each of the three electron beams impacts the phosphor elements of the proper light emitting color. Thus, for example, the electron beam which is modulated with red information impacts the phosphor elements that emit red light. Because the electrons of the beams are charged particles, the earth's magnetic field has an influence on their trajectories which can cause the electrons to impact a phosphor of the improper color, causing misregister, thereby degrading the quality of the image display. For this reason, a magnetic shield, preferably an internal magnetic shield, or IMS, is used within the CRT to act in conjunction with the shadow mask and its frame to shield a substantial portion of the electron beams trajectories from the influence of the earth's magnetic field.
The shielding acts in the following way: the external magnetic field realigns the Weiss domains in the ferromagnetic material of the shadow mask, mask frame and IMS, creating an induced magnetic field that tends to oppose the action of the external field of the earth which has given rise to it. If the material of the ferromagnetic components, i.e., of the shadow mask, mask frame and the IMS, has high magnetic permeability, the induced field at least partially opposes the external field. Hence, the deleterious effect of the earth's magnetic field is reduced.
For more efficient compensation, the ferromagnetic material of the components has to be de-magnetized with a magnetic field that is provided by the degaussing coil, fixed by appropriate means to the rear of the tube and overlying the ferromagnetic components. One such arrangement is shown in FIGS. 1 and 2, in which a CRT 1 comprises a glass envelope having a faceplate panel 2 with a sidewall 3 and a funnel 4 which includes an neck. The sidewall 3 of the panel 2 is attached to the funnel 4 by a frit seal 5. An implosion protection band 6, for example a shrinkfit band, surrounds at least a portion of the sidewall 3 of the panel 2. The band 6 has four mounting lugs 7 located at the corners of the tube to provide a means to affix the tube within a receiver (not shown). A deflection yoke 8 is attached to the funnel 4 of the tube to deflect the electron beams from the electron gun (also not shown) that is located in the neck of the tube. A degaussing coil 9 having two loops 10 and 11 is fixed by appropriate means to the external rear surface of the tube 1. In FIGS. 1 and 2, the coil 9 is looped over the mounting lugs 7 and secured by means of a pair of holding straps 12. Alternatively, as shown, in FIGS. 3 and 4, the coil 9 may be secured to the band 6 by means of the straps 12 or by coil hangers 13 attached to retainers 14 adjacent to openings 15 formed near an edge of the band. The openings 15 are formed by either stamping or piercing the material of the band, as is known in the art. The retainers 14 are then bent out of the plane of the band to accommodate the coil hangers 13.
Possible drawbacks of looping the degaussing coil over the mounting lugs 7 in the manner shown in FIGS. 1 and 2 is that the coil may not be accurately positioned on the tube, or that its position on the lugs may interfere with the attachment of the tube within the receiver.
One drawback of forming the openings 15 in the band 6, for attaching the degaussing coil 9 by means of either the straps 12 or the coil hangers 13, is that the stamping and subsequent forming operations frequently causes burrs, or sharp projections, to be formed on the band adjacent to the openings. Burrs are undesirable because they can either cut the insulation on the degaussing coil 9 or injure anyone handling the tube. Because of the size of the bands and the relative fragility of the retainers 14, any removal of the burrs must be done by hand. This adds substantial labor cost to the price of the bands. Additionally, it is difficult to accurately form and locate the retainers 14 on the band. Also, bands with formed retainers pose shipping and handling problems, because special packing and handling precautions are required to prevent damage or distortion of the retainers. Despite the use of protective coatings on the bands to prevent rust, the stamping and retainer-forming operations expose the underlying ferromagnetic material of the band to the atmosphere, thus providing sites where rust may form. Rust is not only cosmetically undesirable but also may be a source of defects in subsequently applied faceplate coatings, which are formed by spraying suitable materials onto the viewing faceplate and adjacent portions of the panel sidewall, including the band, of evacuated CRT's.
Thus, it is desirable to provide attachment means for use with an implosion protection band that have none of the shortcomings or drawbacks of the prior art. Additionally, an implosion protection band made without formed retainers therein may utilize narrower material and is, therefore, inherently less expensive than a conventional band with formed retainers.